RS virus infection have concentrated on three approaches, namely, temperature-sensitive mutants, cold-adapted mutants, and bovine respiratory syncytial (BRS) virus. The most attenuated of the ts mutants was ts-2 (complementation group B). This mutant did not induce any symptoms but unfortunately it had low infectivity for seronegative chimpanzees and young children. Despite many passages at low temperature (24-26 degrees C) cold-adapted mutants, produced in our laboratory and in the Sandoz Laboratories (Austria), retained virulence for chimpanzees equivalent to that of wild type virus. Bovine RS (BRS) virus, a naturally-occurring bovine pathogen, was not pathogenic for experimental rodents or primates. Studies in owl monkeys and chimpanzees, however, showed the virus to have very low infectivity, and limited immunogenicity.